After the first stage compression, the air-intake temperature of the second stage is decreased by strengthening oil-injected cooling. The compressed air is isobaric compressed and thus reduced the compression power consumption. Each stage compression ratio is greatly decreased compared with the single stage compression because of the design of isobaric compression ratio. Rotated leakage between rotors is also greatly reduced, thus the volumetric efficiency and adiabatic efficiency are greatly increased. This accordingly improved the air capacity. The air capacity of two-stage cccompressors is  15% more than single stage compressors and can save energy as much as 15%.